The present invention relates to a rotation rate measuring instrument. An instrument of this kind is described in an article by J. L. Davis and S. Ezekiel, "Techniques for Shot-Noise-Limited Inertial Rotation Measurement Using a Multiturn Fiber Sagnac Interferometer", Proceedings of the Society of Photo-Optical Instrumentation Engineers, Volume 157, Laser Inertial Rotation Sensors, Aug. 30-31, 1978, San Diego, Calif. pages 131-136.